Rubber and similar products, such as neoprene, urethane, and the like are ubiquitous in modern life. They are used in thousands of places considered entirely normal today. The properties of rubber, including elasticity, the ability to be easily foamed, and the ability it was named for, the ability to erase pencil lines, have been exploited thoroughly in modern life. Of course, since World War II and after thousands of synthetic latexes have been formulated to fill various specialized niches. However, despite all these improvements, rubber is poor at the absorption of moisture and water.
Rubber is normally quite water repellant. It can be made to absorb moisture if it is foamed, one example being kitchen sponges, but the moisture goes to the interstices of the rubber rather than into the rubber composition itself. It would be advantageous to have a rubber composition that absorbed at least some moisture as well as odor causing components, such as ammonia and the like.
One example of a use for this would be for use in the inner soles placed in athletic shoes and the like. Feet, as other parts of the body, produce perspiration. This creates a continuous source of moisture on the interior of shoes. Since a foot confined in a shoe has little opportunity to shed this moisture, there is a problem of keeping the feet fresh. A variety of methods, for example powders and the like, have been tried to keeping shoes smelling fresh and clean. One other method of preparing shoes for wearing includes the addition of a shoe insert to the interior of a shoe. These inserts frequently have ingredients that allow the foot and the
One potential solution is the addition of zeolites to the rubber composition. Zeolites are alumino-silicates having complex crystalline structures than can allow the crystalline structure to absorb small molecules, such as water ammonia. Zeolites are also used in cracking catalysts for petroleum processing and similar uses. Again, the reason for such uses is that zeolites can exclude molecules based on the molecular size of the molecule. They can exclude in the micro range (<2 nm) or the meso-range (2-20 nm).
There are families of zeolites or molecular sieves as they are frequently known, that are defined by their molecular structures, in particular their crystalline structures, and their chemical compositions. One important family of zeolites is alumino-silicates having the general empirical formula of M2nO.Al2O3y.SiO2w.H2O, where y is 2 or greater, M is the charge of a balancing action, such as sodium, potassium, magnesium and calcium, n is the action valance, and w represents the number of moles of water contained in the zeolitic voids.
Of the 120 or so zeolites known to exist, about 50 are naturally occurring and the rest are synthetic. Commercially available naturally occurring zeolites include chabzite, eroionite, mordenite, and chinoptilolite. Other zeolites included faujasite, philipsite, zeolite A, zeolite L, Zeolite Y, zeolite X and ZSM-5. All these zeolites vary by the particulars of their cage structure and the pore sizes, and therefore, vary in the molecules that can be excluded by the zeolites. Many zeolites are mined directly from the earth, and these are less pure than synthetic zeolites containing dirt, clays, and other associated material from the earth. However, mined zeolites work well in this application, as long as they are ground to at least as small as 300 mesh. And that is the source of their problem for use in rubber. If the zeolite is added just as mined, the final product will be brittle, and will not absorb water or moisture.